Front projection screen with lenticular front surface

ABSTRACT

A front projection screen is made up of a front portion and a rear portion, the front portion having a front surface lenticular lens array and light scattering particles forming a diffusion region between the front and rear surfaces, and the rear portion having a reflective surface spaced apart from the diffusion region. In use, an incoming ray of projected light traverses the diffusion region and the space between the diffusion region and the reflective surface, before being reflected back through these regions again. Thus, the ray traverses different portions of the diffusion region in different directions, increasing the likelihood that the ray will encounter at least one scattering particle, thus reducing the incidence of speckle. A stiffening layer of a glass plate with an anti-reflection coating is bonded to the front surface of the screen.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 08/310,195, filedSep. 21, 1994, now U.S. Pat. No. 5,473,469, issued on Dec. 5, 1995,which was a continuance-in-part of application Ser. No. 08/241,836,filed May 12, 1994, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to front projection screens, and moreparticularly relates to such screens having a lenticular front surface.

The development of small display devices such as LCDs (liquid crystaldisplays) and DMDs (deformable mirror devices) suitable for reproducingvideo images, has given rise to the need for small aperture projectionoptics to project these images, and for front projection screens todisplay the projected images with adequate contrast over a wide viewingangle.

Various front projection screens are known which employ either a rearsurface lenticular lens array or a linear fresnel lens array, togetherwith light scattering particles to spread the image into the audiencefield. See, for example, U.S. Pat. Nos. 4,767,186; 4,964,695; and4,911,529. These screens may also employ light-absorbing areas betweenthe lens elements to absorb ambient light and thus improve imagecontrast.

One problem encountered with such small aperture systems is that lightmay pass through the screen without encountering any of the lightscattering particles, giving rise to a phenomenon known as scintillationor speckle, which is the random occurrence of bright spots on thescreen. Such scintillation or speckle is obviously undesirable in thatit reduces definition, and is distracting to the viewer.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention is to provide a frontprojection screen in which the incidence of scintillation or speckle isreduced.

In accordance with the invention, a front projection screen is made upof a front portion and a rear portion, the front portion having alenticular lens array on its front surface, and incorporating lightscattering particles to form a diffusion region between the front andrear surfaces, and the rear portion having a light reflective surfacespaced apart from the diffusion region. In use, an incoming ray ofprojected light traverses the diffusion region and the space between thediffusion region and the reflective surface, before being reflected backthrough these regions again. Thus, the ray traverses different areas ofthe diffusion region in different directions, increasing the likelihoodthat the ray will encounter at least one light scattering particle, andthereby reducing the incidence of speckle.

As used herein, the term "lenticular lens array " means an array ofmutually parallel elongated lens elements or lenticules, extendinglongitudinally across the screen in one direction, and having a definingcontour in the transverse direction.

The diffusion region can extend to the rear surface of the frontportion, e.g., across the entire thickness of the front portion from thefront surface to the rear surface, with the rear portion providing thespace between the diffusion region and the reflecting surface. Instead,the diffusion region may be spaced apart from the rear surface of thefront portion, e.g., may be confined near the front surface, in whichcase the remaining region between the diffusion region and the rearsurface of the front portion can function as a spacer region between thediffusion region and the reflecting surface. The reflective surface maythen be located at the rear surface of the front portion.

In addition, the rear surface of the front portion may define a secondlenticular lens array, parallel to the first array, with each lenticularlens element or lenticule of the first array having a correspondinglenticule in the second array.

In accordance with another embodiment of the invention, the front and/orthe rear lenticular array is made up of lenticules having steepsidewalls, so that at least some portion of the light entering theselenticules is totally internally reflected by the sidewalls andconcentrated in the tip region of the lenticule. This total internalreflection (TIR) is described in more detail in the above-referencedU.S. Pat. Nos. 4,767,186 and 4,964,695, the specifications of which areincorporated herein by reference.

In any of the above embodiments and variations of embodiments, a patternor matrix of light-absorbing material may be present between thelenticules of the front and/or rear lenticular lens array, in order toenhance the contrast of the image displayed in the presence of ambientlight.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail with reference to anumber of specific embodiments, as illustrated in the accompanyingdrawing, in which:

FIG. 1 is a schematic cross section of one embodiment of the screen ofthe invention, having a front portion with a front lenticular array, alight absorbing structure associated with the array, and bulk diffusion,and a rear portion having a rear reflective surface;

FIG. 2 is a schematic cross section of another embodiment of the screenof the invention, similar to that shown in FIG. 1, except that the frontportion also has a rear lenticular lens array;

FIG. 3 is a schematic cross section of another embodiment of theinvention, similar to that of FIG. 1, except that the diffusion regionis confined near the front surface, and the rear portion has a frontreflecting surface;

FIG. 4 is a schematic cross section of still another embodiment of theinvention, similar to that of FIG. 3, except that the front portion alsohas a rear lenticular lens array;

FIG. 5 is a schematic cross section similar to that of FIG. 4, exceptthat the reflective layer follows the contour of the rear lenticulararray;

FIG. 6 is a schematic cross section similar to that of FIG. 5, exceptthat the reflective layer is confined to the tip regions of the rearlenticules;

FIG. 7 is a schematic cross section similar to that of FIG. 2, exceptthat a spacer region is present in the front portion;

FIG. 8 is a schematic cross section of a front projection screen similarto that of FIG. 2, except that it lacks the spacer layer;

FIG. 9 is a schematic cross section of a front projection screen similarto that of FIG. 2, except that the light absorbing structure isassociated with the rear lenticular lens array; and

FIG. 10 is a schematic cross section of a front projection screensimilar to that of FIG. 1, including a stiffening front transparentlayer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, screen 10 is composed of front portion 12 andrear portion 14. Front portion 12, of a light transmissive material,such as polymethyl methacrylate, has a front surface 16 defining alenticular lens array 17, made up of individual lenticules orientedparallel to one another and extending longitudinally across the screen.FIG. 1 is a cross section taken transverse to the longitudinaldirection, showing the contour of the lenticules. The front surface alsohas a light absorbing structure including raised ribs 18, extendingbetween the lenticules, and a light absorbing layer 20 on top of theseribs. This layer 20 is intended to absorb a large portion of lightstriking the screen at relatively large angles (e.g., ambient light)while absorbing only a small portion of the light striking the screen atsmall angles (e.g., projected light).

The first portion 12 has light scattering particles dispersed throughoutthe bulk of the portion, to form diffusion region 22.

The second portion 14 of screen 10 is made up of spacer layer 24 of alight transmissive material, such as glass, and layer 26, forming alight reflective surface 28. Layer 28 may be of any suitablemirror-forming material, such as silver or aluminum.

The function of spacer layer 24 is to provide a space between thediffusion region 12 and the reflective surface 28. Thus, an incominglight ray 29 passes through diffusion region 22, then through spacerlayer 24, then is reflected back through these layers again beforeexiting the screen. This arrangement results in significantly lessspeckle than is seen in the front projection screen shown in FIG. 8.

FIG. 2 shows another embodiment of the invention, similar to theembodiment of FIG. 1, except that front portion 32 of screen 30 has asecond lenticular array 50 on the rear surface. This lenticular array isalso composed of mutually parallel, longitudinally oriented lenticules,each lenticule opposed to a corresponding lenticule of the frontsurface. Thus, the pitch p (distance between adjacent lenticules of anarray) is the same for both the front and the rear arrays, while thecontour of the lenticules of the second array may be different from thecontour of the first array.

As in the embodiment of FIG. 1, a ray 52 traverses the bulk diffusionregion 42 twice, the second traverse being after passage through spacerlayer 44 and reflection from surface 48, resulting in significantly lessspeckle than is seen in the screen of FIG. 8.

FIG. 3 shows another embodiment of the invention, similar to theembodiment of FIG. 1, except that the light scattering particles ofscreen 60 are confined to a diffusion region 72 near the front surfaceof front portion 62, and that rear portion 64 is composed of layer 74,which forms a light reflective surface 76 where it meets front portion62. In this configuration, the spacer layer 24 of FIG. 1 is replaced bythe spacer region 78 in front portion 62, which region 78 does not haveany light scattering particles. Thus, ray 79 upon entering the screen,traverses diffusion region 72, then spacer region 78, then is reflectedfrom surface 76, to again traverse spacer region 78 and diffusion region72, resulting in a reduced incidence of speckle when compared to thescreen of FIG. 8.

In the FIG. 3 embodiment, the layer 74 may be of substantial thickness,to provide mechanical support for the front portion, or may be arelatively thin layer, for example, of a coated or vapor depositedmaterial, on the rear surface of the front portion.

FIG. 4 shows yet another embodiment of the invention, similar to theembodiment of FIG. 3, in which the front portion 82 of screen 80 has asecond lenticular surface 84, similar to lenticular surface 50 of FIG.2. In this embodiment, incoming ray 86 traverses diffusion region 88,then spacer region 90, then is reflected from surface 92, then againtraverses spacer region 90 and diffusion region 88 before exiting screen80, resulting in a reduced incidence of speckle when compared to thescreen of FIG. 8.

FIGS. 5 and 6 show two different variations of the embodiment of FIG. 4.In FIG. 5, screen 100 is composed of front portion 102, similar to frontportion 82 of FIG. 4, and rear portion 104, consisting simply of a layerof material 106 which forms reflective surface 108 conforming to thecontour of second lenticular surface 110. Layer 104 may, for example, bea vapor deposited layer of silver or aluminum. Thus, ray 112 uponentering screen 100, traverses diffusion region 114 and spacer region116, is reflected at surface 108, and again traverses spacer region 116and diffusion region 114, before exiting the screen, resulting in areduced incidence of speckle when compared to the screen of FIG. 8.

In FIG. 6, screen 200 is composed of front portion 202, similar to frontportion 102, and rear portion 204, now reduced to separate layers of amaterial 206 confined to the tip portions 212 of the lenticules 220 ofsecond lenticular surface 210. Material 206 may be, for example, silveror aluminum, vapor deposited through a mask, or coated onto the tipswith an applicator such as a roller, having a much larger dimension thanthat of the lenticules. Thus, ray 216 upon entering screen 200,traverses diffusion region 218 and spacer region 220, is reflected atsurface 208, and again traverses spacer region 220 and diffusion region218, before exiting the screen, resulting in a reduced incidence ofspeckle when compared to the screen of FIG. 8.

In FIG. 7, screen 300 is composed of front portion 302 and rear portion304, similar to front and rear portions 32 and 34 of FIG. 2, except thatin front portion 302, the diffusion region is confined to a region 306near the front surface, leaving a spacer region 308 which is free oflight scattering particles. Rear portion 304 is similar to rear portion34, composed of spacer layer 310 and layer 312 forming reflectivesurface 314. Thus, incoming ray 316 traverses diffusion region 306,spacer region 308 and spacer layer 310, before being reflected bysurface 314, to re-traverse these regions and layer before exiting thescreen, resulting in a reduced incidence of speckle when compared to thescreen of FIG. 8.

The embodiments of FIGS. 2, 4, 5, 6 and 7 are particularly advantageousin that commercially available rear projection screens having surface orbulk diffusion can be employed as the front portion of the frontprojection screen of the invention. Such screens are described, forexample, in JEI, December 1993, pp. 27, 33.

FIG. 8 shows a front projection screen 400, having a front portion 402of such a prior art rear projection screen, having a front lenticulararray 404, raised portions 406 with light-absorbing layer 408, rearlenticular array 410 and bulk diffusion 412. The rear portion consistsof mirror 403. Thus, the screen lacks the spacer layer 24 of FIG. 1.

The screens of FIGS. 2 and 8 were visually compared for focus, moire andspeckle, for different thicknesses of the spacer layer 44. Results areshown in the following Table.

                  TABLE                                                           ______________________________________                                        Spacer layer Focus       Moire     Speckle                                    ______________________________________                                        none   (FIG. 8)  best        high    high                                     3/32   inch      good        medium  low                                      1/8    inch      moderate    low     low                                      1/4    inch      poor        low     lowest                                   ______________________________________                                    

FIG. 9 shows a front projection screen 500, having a front portion 502and a rear portion 504, similar to the front and rear portions 32 and 34of FIG. 2, except that in front portion 502, the light absorbingstructure is associated with the rear lenticular lens array 510, insteadof the front array 507. Thus, ribs 512 and light absorbing layers 514are located between lenticules 511, and any light reaching such layers514, such as ray 524, is absorbed.

FIG. 10 shows a front projection screen 1000, having front and rearportions 12 and 14, identical to those in FIG. 1, and a stiffening fronttransparent layer 17, including a plate 13, and an anti-reflection layer15. Plate 13 may be of a plastic or glass composition, for example,while layer 15 may be a 1/4 wave thick MgF₂, for example, where thewavelength is the central or dominant wavelength of the projected light.Such a layer 17 tends to prevent the delamination of front and rearportions 12 and 14, caused for example, by a humid ambient.

As will be appreciated, the front and rear portions 12 and 14 may bereplaced by the front and rear portions of any of the other embodimentsdescribed herein.

The invention has been described in terms of a limited number ofembodiments and variations of embodiments. Other embodiments andvariations will become readily apparent to those skilled in the art, andare intended to be encompassed within the scope of the invention asdefined in the appended claims.

What we claim as our invention is:
 1. A front projection screen having afront portion and a rear portion, each portion having a front surfaceand a rear surface, the front portion having light scattering particlesdefining a diffusion region between the front and rear surfaces, thefront surface of the front portion defining a front lenticular lensarray, the array comprised of mutually parallel lenticules extendinglongitudinally across the screen, and the rear portion defining areflective surface in spaced apart relationship to the diffusion regionof the front portion, and a front transparent stiffening layer, thestiffening layer comprising a plate having a front and a rear surface,the rear surface of the plate in contact with the front surface of thefront portion defining the lenticular lens array, and the front surfaceof the plate having an anti-reflection layer.
 2. The front projectionscreen of claim 1 in which the plate is glass and the anti-reflectionlayer is MgF₂.
 3. The front projection screen of claim 2 in which thethickness of the anti-reflection layer is 1/4 of the central wavelengthof the projected light.